The invention relates to a filling-level indicator for fuel tanks and comprising a sonic transmitter, a sonic receiver, and a transmitting body extending into a fuel tank. It is already known to introduce sound into a sound-conducting solid body and determine the variation in the propagation velocity (phase velocity) of flexural waves with a receiver. The propagation velocity of the flexural waves in the solid body is a function of the height of the fluid level. The propagation velocity in the solid body in an empty fluid vessel serves as a reference value. Additional measurements are necessary to obtain this function. Furthermore, the transmitter and receiver should not come in contact with the fluid, otherwise disadvantages arise due to deficient sealing of the transmitter or the receiver or inadequate resistance of the points of adhesion. Also, the zero point and the measuring sensitivity of the filling-level indicator are temperature-dependent.